1. Field of the Invention
The subject invention relates generally to the field of communications and more particularly to a telephone line interface circuit capable of complying with various impedance requirements worldwide.
2. Description of Related Art
A telephone line interface circuit must have a specified DC resistance and AC impedance when connected to a telephone network, which in general have different values. For example, the Federal Communications Commission in the United States specifies a DC resistance of 300 ohms or less, and an AC impedance equivalent to a resistive 600 ohms. Most countries worldwide adopt these specifications, but a number of countries instead require a complex network for AC impedance.
A conventional line interface circuit generally includes an electronic inductor (or xe2x80x9cgyratorxe2x80x9d) and a passive network. The electronic inductor determines the DC resistance of the circuit, whereas the passive network determines its AC impedance.
A typical electronic inductor circuit is shown in FIG. 1. Resistors R1 and R2 form a voltage divider with respect to a DC line voltage VTR, and set a DC bias voltage VB at the base of transistor Q1. A bypass capacitor C1 filters out any AC signals Vb that result from an AC signal Vtr on the telephone line voltage. The current IC through the transistor Q1 is approximately equal to the ratio of emitter voltage VE over the emitter resistor RE. The emitter voltage VE can be calculated as VE=VBxe2x88x920.7V. Since the AC voltage at VB is completely filtered out by capacitor C1, the current through the transistor Ql will be only DC. If resistors R1 and R2 are chosen to be large, the current through the bias network becomes negligible with respect to IC, and the line current ITR is approximately equal to IC. The effective impedance of the electronic inductor, therefore, can be obtained as the ratio of VTR over ITR. It is apparent from the latter equation that the DC resistance is a finite value, whereas the AC impedance approaches infinity because the AC current Itr through the electronic inductor is close to zero. Hence, the circuit is called an xe2x80x9celectronic inductorxe2x80x9d since it passes DC, but blocks AC.
A typical passive network used as AC impedance in a telephone line interface circuit is shown in FIG. 2. In its simplest form, this network is comprised of a 600-ohm resistive load R3 in series with a large coupling capacitor C2, generally 100uF or higher. The network is terminated to AC ground and is effectively in parallel with the electronic inductor of FIG. 1. In this configuration, an incoming AC signal from the telephone line xe2x80x9cseesxe2x80x9d a resistive impedance equivalent to the value of R3. In general, the resistive load R3 is required to be a complex impedance X having the configuration shown in FIG. 3. This complex network is required in Europe and certain other countries worldwide.
In modem applications, such as a computer modem, it is desirable that a line interface circuit complies with the impedance requirements of multiple countries worldwide. Otherwise, a different line interface circuit would have to be designed for use in every country. One solution is to use a plurality of passive networks with switches that select the impedance specific for the country where the circuit is expected to operate, as shown in FIG. 4. Based on a country code entered by the user, a microcontroller (not shown) enables the impedance desired through control lines IS1, IS2, and IS3, which enables a 600-ohm resistive impedance, a complex network X, or any arbitrary impedance network Y, respectively.
The circuit just described does not include a line driver to transmit signals to the telephone line. Two configurations incorporating a line driver TXA are shown in FIG. 5 (A) and FIG. 5 (B), respectively. In FIG. 5 (A), the AC ground of the switches is connected to the output of the transmit driver TXA, which is effectively an AC ground with respect to the line signal. In FIG. 5 (B), the transmit driver TXA transmits signals to the line through the electronic inductor circuit, specifically through the base of the transistor Q1.
The circuits just described exhibit a number of drawbacks, especially when considered in the context of lightning protection requirements. First, the coupling capacitor C3 of the impedance network is too large and expensive. Depending on the protection circuit used, the rating required of this capacitor could be as high as 300V. Additionally, the 600-ohm and other worldwide complex impedance networks are too small when compared to lightning voltages, resulting in excessive current that must be adequately limited with other protection devices. Furthermore, the impedance selection switches SW1, SW2, and SW3 are expensive because they must satisfy high voltage and current ratings as well. Lastly, in the configuration of FIG. 5 (A), the transmit driver TXA must have a relatively low output impedance in order to drive the 600-ohm resistive load and the line impedance directly.
Using a telephone line interface circuit with negative feedback makes it possible to control the current through the circuit so that the ratio of the voltage applied across the circuit to the current is the desired value of impedance. Voltage feedback from the line controls the current through the circuit, thereby reflecting a xe2x80x9cvirtualxe2x80x9d impedance to the telephone line. By adequately filtering the voltage feedback, the reflected impedance to the line can be adjusted to any desired impedance function, without using the actual values of the impedance.
In a preferred embodiment, a selectable impedance network is included to provide for worldwide compatibility in a single circuit. The impedance network may be implemented using either passive or active components. The transmit signal is preferably applied to a positive input of an operational amplifier connected to the base of the electronic inductor transistor.